The present invention relates to a multi-modal handy phone provided with a plurality of radio modules for enabling the handy phone to select and communicate through a radio telephone network among those having different systems such as the PHS (Personal Handy phone System) and the PDC (Personal Digital Cellular) system, for example, optionally or dealing with degradation of communication quality.
Compared to the radio telephone network called the PDC which has a wide service area in Japan covering most of the principal cities, the service area of the PHS as of 1995 is yet smaller, only covering busy streets and shopping areas around railway stations in metropolises such as Tokyo. With respect to the covering area of a relay station, that of the PHS is limited to several hundreds' meters in radius, while that of the PDC attains several kilometers. However, there are many underground facilities such as a sub-mall or a sub-street where the PHS is available even when the PDC system is not accessible. The low cost service charge is also an advantage of the PHS system superior to that of the PDC system.
This is the reason why the multi-modal handy phone is expected. With a multi-modal handy phone having radio modules accessible to different radio telephone networks, a user may select another network that remains available even when the network in use goes out of service because the user has gone out from its service area, for instance.
As a prior art similar to the multi-modal handy phone, there is a Japanese patent application laid open as a Provisional Publication No. 274552/'86, wherein is disclosed a communication apparatus which can automatically call the other party through another network when a communication network is found unavailable, effectively increasing connection probability with the other party.
However, any countermeasure against disconnection during a communication because of going out of the service area, for instance, is not considered in the prior art being proposed to be applied for wire-networks.
In another Japanese patent application laid open as a Provisional Publication No. 152745/'94, a prior art concerning a portable telephone system is disclosed, wherein an exchange station for a private radio telephone network in a precinct is provided with a relaying apparatus of a public radio telephone network to the private radio network having different carrier frequency than the public network. The relaying apparatus receives data from the public network for transmitting the data to a mobile terminal of the private network for enabling a user of a portable telephone of the public network, coming into the precinct from out of the service area of the public network, to continue receiving the data through the mobile terminal of the private network.
If the latter prior art could be easily applied between different public radio telephone systems such as the PHS and the PDC system, a user of a PHS handy phone, for example, might be able to receive a call from the PDC system with his ordinary PHS handy phone through a relay station of the PHS provided with a relaying apparatus of the PDC system. However, an enormous cost would be needed in reality together with intricate operation for providing such a relaying apparatus for every relay station of the PHS, for example.